For heat-treatment processes (diffusion, oxidation and the like) of semiconductor wafers, normally a tube or furnace is used into which the wafers are introduced on a crucible. For mass production, it is of vital importance that all the wafers be subjected to the same conditions within the tube. This means that the temperature characteristic (temperature profile) over the length of the tube must be as far as possible constant.
In order to obtain this constant temperature profile, hitherto tubes have been used which are loaded and unloaded through the same end. If the tubes are loaded at a low "entry rate", something which is essential particularly when the semiconductor wafers are of large diameter, then inevitably differences in the temperature-time characteristics occur, and these give rise to a major degree of spread in the electrical parameters.
To achieve higher furnace capacities and more uniform temperature-time characteristics, already the possibility has been suggested of using several conveyor baffles or crucibles in a continuous process. In this context, the tube is loaded at one end and unloaded at the other. This, however, gives rise to high friction forces between the conveyor baffles and the wall of the tube. Furthermore, a tube made of quartz will bend at the high temperatures prevailing, due to the loading imposed by the conveyor baffles carrying the semiconductor wafers. Finally, the contact with the conveyor baffles produces nonuniform heating of the semiconductor wafers.